Wow, what a dilemma! My first thought is that the energetic boxer and guinea pigs wouldn't necessarily be a good mix. I know this might sound odd, but I tend to view guinea pigs as more of an "adult" pet than a child's pet - I know that's not how they are seen, but their personalities are more "subtle" than say a cat or dog or even a budgie I think, and do require a fair bit of looking after, which will grow tiresome if they don't really "light your fire."
In your situ, I would lean towards a smallish dog (but anything with border collie in it will give your Boxer a run for its money in the energy stakes, so are you sure you want another?) along the lines of a Jack Russell or Border Terrier, which doesn't fit the profile of "yappy toy dog" (I'm a small dog fan myself, but I know many aren't,) but is less demanding food, space, poop and exercise wise - and smaller dogs tend to live longer than bit ones. I've seen some Jack Russell crosses in rescues, so you might strike it lucky.
Ferrets also sound a good idea, but might not be a good match with your chickens! Cats actually are generally pretty okay with chickens, so I've heard.
I think it's probably as important to find a pet that fits with your other pets as it is to fit with your DD!
(We've been dithering for a couple months about getting a dog for the pigs, and in the end opted for a Dog Cat - a Maine Coon, like in the "Be More Dog" commercials. So far she's "interested" in the pigs, but thankfully not in a culinary way!)